Characterization of and Structural Insight into Struvite-K, MgKPO₄·6H₂O, an Analogue of Struvite

Struvite-K (MgKPO₄·6H₂O) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state ²⁵Mg and ³⁹K MAS NMR techniques, alongside ³¹P MAS NMR and thermal analysis. A single resonance was present in each of the ²⁵Mg and ³⁹K MAS NMR spectra, reported here for the first time alongside the experimental and calculated isotropic chemical shifts, which were comparable to the available data for isostructural struvite (MgNH₄PO₄·6H₂O). An in situ high-temperature XRD analysis of struvite-K revealed the presence of a crystalline-amorphous-crystalline transition that occurred between 30 and 350 °C, following the single dehydration step of struvite-K. Between 50 and 300 °C, struvite-K dehydration yielded a transient disordered (amorphous) phase identified here for the first time, denoted δ-MgKPO₄. At 350 °C, recrystallization was observed, yielding β-MgKPO₄, commensurate with an endothermic DTA event. A subsequent phase transition to γ-MgKPO₄ was observed on further heating, which reversed on cooling, resulting in the α-MgKPO₄ structure stabilized at room temperature. This behavior was dissimilar from that of struvite exposed to high temperature, where NH₄ liberation occurs at temperatures >50 °C, indicating that struvite-K could potentially withstand high temperatures via a transition to MgKPO₄.